1. Field of the Invention
The present invention relates to a focal point detection device for use in a projection aligner for semiconductor manufacturing, for transferring a circuit pattern onto a wafer.
2. Description of the Related Art
There are several known surface position detection devices for use in a projection aligner. For example, Japanese Unexamined Patent Publication No. 62-299716 (U.S. Pat. No. 4,748,333) discloses one height detection system that measures the center of an exposure area on a wafer. In another device disclosed in Japanese Unexamined Patent Publication No. 2-102518 filed by the applicant of the present invention, a plurality of height detection systems are provided on an exposure area on a wafer and based on a plurality pieces of height information, the inclination and height of the exposure area are calculated and adjusted.
As seen from these known systems, most of the present surface position detection devices employ a laser, an LED or the like as a light source.
Generally speaking, in an optical system, an image of a slit illuminated by a light source is obliquely directed to a wafer to be detected using a projection imaging optical system, the slit image reflected off the wafer is re-focused on a position detection element using a image receiving optical system, and the up and down movement of the wafer to be detected is measured in the form of motion of the slit image on the position detection element.
In the optical system, resist should be applied to the surface of the wafer to be detected to the extent that the wafer is flat enough to be considered as a mirror wafer. By setting the incident angle of incident light to be 70 degrees or greater, the reflectance at the flattened resist surface of the wafer is increased to measure the resist surface position of the wafer.
A wafer has a stepwise or uneven profile with a diversity of patterns thereon.
In an integrated circuit portion in a large scale integrated circuit (LSI) chip where fine patterns are formed, the applied resist fills and smoothes the unevenness of the patterns to the extent that the resist surface is made flat enough to be considered as an optical mirror surface.
However, a large step is formed in the vicinity of a scribe line between LSI chips, and the application of resist is still unable to sufficiently fill the notch of the step.
The surface of the applied resist in the vicinity of the scribe line thus suffers dents (or projections).
The resist surface of the wafer thus has both areas considered as an optical mirror surface and other areas having an uneven profile.
As already described, when a slit image is directed to a flat resist surface considered as a mirror surface, an incident light ray is reflected at a substantially constant angle of reflection, and an image receiving optical system finely re-focuses the slit image on a position detection element.
In contrast, when the slit image is directed to the uneven resist surface, the light ray is reflected in a direction different from that reflected from the flat resist surface, and becomes a deflected light ray.
When the reflected light ray is deflected to the image receive optical system, the reflected light ray flux is fully or partially shaded, presenting difficulty in focusing the slit image on the position detection element.
Specifically, the symmetry of the re-focused slit image is destroyed, or the slit image is entirely missing. As a result, detection or measurement accuracy may be degraded, or measurement itself may become impossible.
To cope with this problem, known surface position detection devices use an oversize projected slit image to decrease the ratio of the unevenness profile area to the flat area within the reflected slit image coverage over the wafer surface or use an elongated rectangular slit image that is projected at an orientation of 45 degrees relative to the LSI chips on the wafer so that the scribe line area having an unevenness profile is relatively small with respect to the entire area of the slit image. Thus, the degree of shading through the image receiving optical system is reduced.
However, the design of LSI circuitry changes the ratio of the unevenness profile area to the entire resist surface. Thus, there is a possibility that, depending on the process used to design the LSI circuitry, the above methods are still subject to detection accuracy degradation leading to detection failure.